Corrosion resistant ferritic stainless steel

ABSTRACT

A ferritic stainless steel resistant to pitting and crevice corrision and containing in addition to iron, correlated amounts of chromium, molybdenum, silicon, nickel and metal from the group consisting of titanium, vanadium and columbium. Other constituents can also be present.

United States Patent 1191 Bieber 1451 Sept. 24, 1974 CORROSION RESISTANT FERRITIC STAINLESS STEEL [75] Inventor: Clarence George Bieber, West Milford, N.J.

[73] Assignee: The International Nickel Company,

Inc., New York, NY.

22 Filed: Dec. 23, 1971 21 'Appl. No.: 211,740

Related US. Application Data 63] Continuation-in-part of Ser. No. 841,138, July 11,

1969, abandoned.

[56] References Cited UNITED STATES PATENTS 2,051,415 8/1936 Payson 75/128 w 2,083,524 6/1937 Payson 75/128 W 2,110,891 3/1938 Reitz 75/126 C 2,141,016 12/1938 Payson 1 75/128 W 2,183,715 12/1939 Franks 75/126 C 2,518,715 8/1950 Payson 75/126 2,624,671 1/1953 Binder 75/126 C 3,152,934 1/1964 Lula 1 75/128 W 3,177,577 4/1965 Fujimura 1 75/128 T 3,567,434 3/1971 Richardson 75/128 W Primary Examiner- Hyland Bizot Attorney, Agent, or FirmEwan C. MacQueen; Raymond J. Kenny [5 7 ABSTRACT A ferritic stainless steel resistant to pitting and crevice corrision and containing in addition to iron, correlated amounts of chromium, molybdenum, silicon, nickel and metal from the group consisting of titanium, vanadium and columbium. Other constituents can also be present.

13 Claims, N0 Drawings CORROSION RESISTANT FERRITIC STAINLESS STEEL This application is a continuation-in-part of application Ser. No. 841,138 filed July 11, 1969 now abandoned.

As the metallurgist is aware, stainless steels since their inception have had a tremendous impact in virtually all segments of commercial and industrial activity. From small pins to huge vessels, from the most delicate of medical instruments to the most elaborate of chemical equipment, from food and dairy utensils to articles for handling the most acrid of acids and fumes, etc., these steels have played a significant role. And, in terms of the more exotic applications extensive research efforts are being pursued in such fields as aerospace and marine technology, including desalination, oceanography, etc.

Perhaps the principal reason for such diverse utility stems from the ability of stainless steels to resist the destructive influence of various corrosive media. Much has been written in explanation of the possible mechanisms involved, and it appears generally accepted that their corrosion resistant qualities are attributable, at least in part, to the ability to assume the passive condition. This is an inherent phenomenon due, it is be-.

lieved, primarily to composition, mainly chromium, and involves the self-formation of a thin, continuous surface film (mostly chromium oxides) which acts as a protective barrier.

To be sure, all stainless steels do not resist all corrosive environments. One such steel might offer appreciable resistance to acid X but be quite susceptible to acid Y, the opposite being true of a different stainless. And as is so often the case in which practically all members of a class of materials exhibit early or premature failure under a given circumstance, stainless steels in general have proven not to be an exception in certain chloride environments, e.g., stagnant or slowly flowing seawater.

By way of explanation and at the risk of oversimplification, there are any number of different types of corrosion, including general, pitting and crevice, intergranular, stress-corrosion cracking, etc.; however, it is the pitting and crevice form which is of greatest concern herein. As detailed in The Corrosion Handbook (1948) by Uhlig and in a number of other reference works, chloride ions (there are others) seem to display a peculiarly destructive talent in respect of the passive condition referred to above. This destructive manifestation normally occurs at localized surface areas which, for the lack of a better-term, are simply referred to as flaws or imperfections (in operation they serve as natural crevices, hence crevice corrosion").

Needless to say, such flaws are, as a practical matter, impossible to avoid. They may take the form of surface dents, notches, nicks, scratches and the like, defects unintentionally induced. Or they may be inescapably brought about because of the form or shape of a particular article of manufacture stranded cable, cable terminal sockets and valve seats being illustrative. Then, there are the historically troublesome crevices formed by the workings of nature as exemplified by the adherence of barnacles and other marine organisms, etc.

The corrosion breakdown that follows apparently involves the formation of an electrolytic cell in which the localized spot ostensibly serves as a point of concentration for attack by chloride ions whereby an active anode is established, the cathode being a large passive area. The disparity in anode versus cathode area results in a considerable difference in potential which causes a significant flow of current with, as Uhlig puts it, attendant rapid corrosion at the small anode.

When stainless steels are immersed in a chloride environment such as stagnant or slowly flowing" seawater, a not inconsiderable number of them seem particularly vulnerable to crevice corrosion. The protective passive film is deemed, as mentioned above, to be primarily chromium oxide(s), meaning oxygen contributes to the passive condition. Upon rupture of the film if the velocity of seawater in a given case is significant, above say, 3 feet per second (fps), there is a replenishment of oxygen to the ruptured local site such that there is a tendency for the punctured surface to self heal by way of forming new chromium oxide. (Also, rapidly flowing seawater tends to prevent the adherence of barnacles and other organisms). In stagnant seawater, however, such is not always the case due to lack of available oxygen. Moreover, inter alia, the acidity of soluble corrosion products, e.g., ferrous chloride, inhibits restoration of passivity.

It has now been discovered that stainless steels containing chromium, molybdenum and nickel and other constituents as described herein offer greatly improved resistance to the destructive effects of chlorides, provided the constituents are properly correlated in terms of chemistry and provided further they are at least substantially if not completely ferritic in microstructure. Furthermore, the ferritic structure notwithstanding, steels in accordance herewith also manifest good notch toughness qualities, various compositions displaying satisfactory brittle-to-ductile notch toughness even down to temperatures as low as or lower than zero F.

The foregoing combination of characteristics can be contrasted with at least certain of the literature. The Metals Handbook, for example, 8th ed., 1961, pages 554, 558-564, indicates that stainless steels perform well in seawater in the absence of fouling mechanisms (crevices) and if the velocity is 5 fps or greater. Similarly, in Stainless and Heat-Resisting Steels, L. Colombier and J. Hochmann, 2d. ed., (1965), p. 51, the ferritic 16-18 percent chromium grades are said to offer good seawater corrosion resistance provided the water is not stagnant and marine organisms are absent. And as to the known 25-30 percent chromium ferritic stainless steels the authors report them (p. 55) as being objectionable by reason of:

Thus, a steel with 25% chromium melted under normal commercial conditions will give low impact values even at carbon contents as low as 0.03%,

after heat treatment at any temperature between 800C. and 1,200C. and whatever the cooling conditions. This is a most troublesome fact and a major obstacle to the large scale industrial development of the steels. The difficulty is in fact notch sensitivity and nothing else;

In addition to corrosion resistance and toughness, steels in accordance herewith also manifest good tensile strength and tensile ductility, yield strengths as high as 125,000 psi and ultimate tensile strengths upwards of r 160,000 170,000 psi being attainable upon age hardening.

In any case, it is an object of the present invention to provide stainless steels novel in composition and which afford improved resistance to the corrosive effect of various chloride environments.

It is a further object to provide stainless steels having a ferritic or substantially ferritic microstructure and which exhibit satisfactory notch toughness characteristics.

Generally speaking, the present invention contemplates, subject to the qualifications described herein, stainless steels containing (by weight) from 16 to 30 percent, e.g., 18 to 22 percent chromium; from 0.5 up to about 11 percent, e.g., 3 to 1! percent. molybdenum; up to 3 percent, e.g., about 0.1 to 1.75 percent, silicon, the sum of the chromium plus three times the amount of molybdenum plus three times the percentage of silicon (referred to herein as the CMS indicator) being at least about 38 percent; from 2 percent, most advantageously, at least about 3 percent, and up to 8 percent nickel; up to less than 2 percent titanium; up to about 2 percent vanadium; up to about 2 percent columbium; up to 2 percent aluminum; up to 2 percent manganese; and the balance essentially iron.

In carrying the invention into practice, the chromium, molybdenum and any silicon must be correlated if satisfactory hot workability is to be achieved in addition to outstanding corrosion resistance. An alloy containing 20 percent chromium, for example, but molybdenum-free has been found to exhibit inferior corrosion resistance. On the other hand, an alloy containing, say, upwards of 30 percent chromium together with a molybdenum content of 10 percent is at best extremely difficultly, if at all, workable. Thus, where hot workabilityis a prerequisite the CMS" factor should be less than 48 percent and beneficially not more than about 45 percent. Too, while silicon confers resistance to corrosion, it has been found to lessen ductility if present in excess of, say, 1.5 percent or 2 percent. For castings this qualification as to silicon need not obtain and in such instances (or in respect of other applications in which optimum ductility is not of necessity, e.g., deposit of an overlay on a substrate surface) the silicon content can be as high as 3 percent. No significant benefit is derived from higher silicon contents. For applications where it is important to minimize susceptibility to sigma formation, e.g., heavy sections, it is beneficial that silicon not exceed 0.5 or 0.75 percent.

Nickel portrays a most influential role by importantly contributing to high impact strength and low brittle-toductile transition temperatures. Charpy-V notch impact strengths of foot-pounds or greater (a rather often used standard measurement) have been attained at temperatureswell below 100F. It is thought that such a characteristic would tend to mitigate the toughness drawback hitherto characteristic of high chromium ferritic steels. Nickel adds strength to the steels but must, however, be controlled'for if present to the excess various characteristics including workability are impaired. A nickel range of from 3 or 4 percent to about 6 or 7 percent is deemed quite satisfactory.

While either air or vacuum melting techniques can be used in the preparation of the subject steels, at least one element from the group consisting of titanium, vanadium and columbium, particularly titanium, should be present in using air melting processing when the CMS indicator is not more than about 38 percent; otherwise, corrosion resistance suffers as will be illustrated herein. It has been found that a small but effective amount of one or more of these elements can markedly enhance corrosion resistance in various chloride media, e.g., ferric chloride. Should the CMS indicator be appreciably above 38 percent, e.g., 41 or 42 percent or more, air melting can be used in the absence of such constituents, but even under such circumstances (or notwithstanding the utilization of vacuum melting processing), one or more of these elements should be utilized, especially when the most drastic corrosion conditions are expected to be encountered. Furthermore, and apart from good corrosion resistance, in attaining good workability, e.g., forgeability and cold rollability, it is decidedly advantageous that at least titanium be present. The total sum of these constituents should not exceed about 4 percent and a range of about 0.15 percent or 0.2 to about 0.5 perent for each is satisfactory.

Aluminum can be used as a deoxidant but it does tend to impart a dross on the surface of air melted alloys in the molten condition and this can result in ingots with scabby surfaces. It can be present up to 2 percent, e.g., up to 1 percent since it seemingly promotes the effectiveness of the titanium, vanadium and columbium under air melting practice; but it need not exceed 0.1 or 0.2 percent. Actually. it is to advantage to use silicon to lieu of or together with a small amount of aluminum not only for its power in imparting good corrosion resistant qualities but also for deoxidation purposes as well.

As above set forth, it is important that the subject steels be characterized by a ferritic microstructure. Duplex structures such as those comprised of ferrite and a significant amount of austenite are to be avoided since corrosion resistance can be impaired. While a small or incidental amount of, say, austenite can be tolerated, e.g., up to 2, 3 or possibly 5 percent, the approach should be to avoid it. The same applies to phases other than austenite.

In the preparation of the instant steels the use of the purest materials commensurate with reasonable cost, (e.g., ferromolybdenum, ferrochromium and ingot iron) should be employed. Materials of exceptionally high purity, of course, can be utilized, e.g., molybdenum metal pellets, the electrolytic forms of iron, chromium and nickel, etc. As indicated above, both air melting and vacuum processing can be employed. Vacuum melting is, however, preferred since it has been found to promote higher notch-toughness characteristics (both resistance to impact and lower brittle-toductile impact transition temperatures), particularly in respect of columbium-containing steels. In addition to such techniques, other production practices are con templated, including electroslag remelting, continuous casting and powder metallurgical processes.

A suitable pouring temperature is from about 2,800 to 2,900F. As is commonly done in commercial steelmaking practice for wrought products, ingots are preferably stripped hot from the mold and directly transferred to a soaking pit and soaked at a temperature of over 2,lO0 to 2,300F. for a sufficient period and thereupon worked as by forging, hot rolling, etc. The hot finishing temperature can be as low as l,400 to l,500F.

Following hot working, the steels can be annealed at a temperature of l,900 to 2,l00F. prior to cold rolling. Surface defects are removed (e.g., by grinding, polishing) at appropriate intermediate stages but subsequent to final working. A final annealing treatment over the temperature range of 1,900 to 2,100F. or 2,200F. can be used; however, it is much preferred to reheated to 2,200 to 2,300F. before rolling to /8 inch bar, steels A, B, D and 7 being reheated to 1,800F. Steels C, 3, 7, 9-1 1 and 13 were additionally reheated to 2,100 or 2,150F. prior to cold rolling.

use a temperature of 1,950 to 2,050F., e.g., 2,000F., 5 Corrosion tests were conducted using an aggressive since the lower temperature would not only be more corrodent commonly used for test purposes, to wit, an amenable to commercial practice but it is considered aqueous percent ferric chloride solution (Fe C1 that corrosion resistance is likely to be superior. Hold- The test specimens, having a surface area of approxiing periods at the lower annealing temperatures need mately 25 square centimeters, were immersed for not exceed about 10 to minutes. 10 about 72 hours in the 10 percent ferric chloride solu- In order to give those skilled in the art a better appretion, the temperature being maintained at about room ciation of the invention, the following illustrative data temperature (approximately 72F.); however, prior to are given. immersion an intentional crevice was created about the A substantial number of alloys within the invention, surface of the specimens by wrapping a rubber band Alloys 1 through 14, Table I, nominal compositio 15 thereabout. This test being of an accelerated nature is being given were prepared using air or vacuum melt deemed equivalent to an extreme long-time exposure in processing. (A151 316 and Alloys A through E e i seawater and 15 described by M. Stretcher 1n the cluded in Table I for purposes of general comparison). Journal Of the Electrochemical y, VOL pm- An argon cover was used in the vacuum melt process- 375-390, 7, y 9 7 V g V H M ing before adding the deoxldant Calcium-Silicon This The data in Table 1 reflect the excellent corrosion reduced the Vacuum from about 29 a t0 abOut 7 characteristics typical of alloys within the invention. g As to the air melted Steels, Small amount of This is in marked contrast to Alloys A through E and argon was continuously passed through the furnace but Alloy A181 316. In comparision with st l i accop the amount was insufficient to completely prevent OXldance herewith, A151 316 failed catastrophically, undatlon from taking place. dergoing near virtual disintegration. Note might be lngots were Soaked at about t0 2,300013- and taken of Alloy E from which it may be observed that a for the most P thereafter hot rolled Squares nickel content of 10 percent seemed to adversely affect h r 2 X 2 in h or 1% X 1 2in h 0r l X 1 The workability. Thus, it is beneficial that the nickel conspecimens were then cut into two sections one of which tent not exceed 6 or 7 percent, a range of 2.5 to about was retained for stock purposes the other being rolled 3O 6 percent being quite suitable. to /8 inch square bar. A portion of the /8 inch bar was With further regard to Table 1, Alloys A-D are indicremoved for other testing, a part of the remainder ative of what might be expected with low CMS valb ing ho work to about one-fourth inch thi k. ues. As referred to herein, the CMS indicator should ground finished, annealed at 1,900 to 2,200F., cold be at least about 38 percent. Even then, in consistently rolled to about 0.050 inch thick strip and final annealed achieving outstanding resistance to pitting and crevice at 2,000 or 2,100F. Certain of the steels (4-6) were corrosion, particularly in the more drastic corrosive en TABLE 1 Corrosion Behavior. Composition 10% Fe Cl Crevice Wt. Loss. Alloy Cr Ni Mo Ti Al Other Corrosion Mgs.

AlSl 17 12 2.5 Very Heuvy 2.592

A 20 n.a. n.a. 0.5 0.5 0.15 Si-Mn Very Heavy 3.610 B 20 10 n.a 0.5 0.5 Very Heavy 7,910 C 14 n.u. n.a. nu 0.5 0.15 Si-Mn, Heavy 9,000

0.12 C D 26 n.21. n.a 0.5 0.5 0.15 Si-Mn Very Heavy 316 E 20 10 s 0.5 0.25 Broke On Forging l 18 2 8 0.5 0.5 0.15 Si-Mn None None 2 20 5 6 0.5 0.5 None 0.5 3 20 o o 0.5 0.25 0.25 Si Trace None 4 20 6 6 0.5 n.a. 0.35 Si Trace 04 5 20 6 6 0.25 n.a 0.35 Si, None 0.1

0.25 Cb 6 20 6 6 0.25 nu 0.35 Si. None 0.6

0.25 v 7 20 4 8 0.5 0.5 0.15 SiMn None 0.6 8 20 4 8 0.4 0.25 None 0.6 9 20 4 8 0.5 0.5 None 0.6 10 24 2 5 0.5 0.5 Very Slight 1.4 11 24 4 5 0.5 0.25 None 0.7 12 26 5 4 0.5 n.u. 0.25 Si None None 13 28 4 3 0.5 n.21. 0.5 Si none None 14 2x 5 4 0.5 n.21. 0.25 Si None None n.11. not zltldcd Each alloy mell deoxidivcd with (1.05% calcium in the form of a 30% Cu 71 Si master alloy. Balance of each composition was iron plus impurities.

vironments, it is-of benefit that the CMS value be 39 percent or more; however, it is contemplated that where a high level of corrosion resistance is not consistently required, the CMS indicator can be as low as 35 or even down to 34 percent. Stainless steels having CMS" indicators of 38 or 39 percent or more and compositions falling within the following ranges are deemed quite satisfactory: about 18 to 22 percent chromium, from about to percent molybdenum, up to about 2 percent silicon, from about 2.5 to 6 percent 10 nickel, a small but effective amount of at least one constituent from the group consisting of up to 1 percent titanium, up to 1 percent vanadium and up to 1 percent columbium, up to about 1 percent aluminum and the balance essentially iron. Another highly suitable steel contains 22 to 28 percent chromium and 3.5 to 8 percent molybdenum, the remainder of the steel being the same as immediately above.

Steels in accordance herewith have also performed well corrosion-wise in media other than chlorides. Using the same general procedure described in connection with Table I, Alloy 7 was tested in 10 percent I-INO and also in a solution containing 5 percent HNO 5 percent I-I SO and 5 percent I-ICl. In the 10 percent HNO solution, a weight loss of but 0.1 milligram was experienced, the weight loss being slightly more, to wit, 0.7 milligram, in the multi-component solution. No crevice corrosion was detected. It might be added that the magnitude of corrosion resistance afforded by steels within the invention has been such as to render pickling of the steels exceedingly difficult in the annealed condition, e.g., when annealed at 1,950 or 2,000F. or above. Thus, it is to be understood that the alloys of the subject invention can be used in many other corrosive environments apart from chlorides,

To demonstrate the effect of a constituent from the group consisting of titanium, vanadium and columbium on steels prepared by air melting practice, there is reported in Table II the corrosion resistant results obtained in respect of alloys both devoid of and containing titanium. These alloys (including vacuum melted alloys 17 and 18) were prepared and tested in generally the same manner as described in connection with Table I.

Alloys F and G are illustrative of the adverse effects to be expected using air melting processing without an element from the group titanium, vanadium and columbium. As demonstrated by Alloy 19, when the CMS" indicator is appreciably above 38 percent (in this case nominally 40 percent) excellent corrosion resistant characteristics are obtainable using air melting techniques in the absence of each of the elements titanium, vanadium and columbium. However, Alloy 17 of Table II, a vacuum melted alloy, exhibited excellent corrosion resistance in ferric chloride maintained at room temperature. In conducting the same test but only using ferric chloride held at a temperature of approximately 122F., this alloy manifested a loss of 549 milligrams and was noted by heavy crevice corrosion. A similar alloy, to wit, vacuum melted Alloy 18, but which nominally contained 0.4 percent titanium, showed only a loss of 0.4 milligram and very slight crevice corrosion when tested under the same conditions. Thus, as previously indicated, it is quite advantageous that one of these constituents be present.

Notch toughness characteristics are set forth in Table III concerning a series of alloys within (Alloys 20-26) and without (Alloy H) the invention. All were vacuum melted and oil quenched after annealing, Alloys 20-23, being annealed at 2,100F., the others at 2,000F. Liquid quenching has been found to offer better notch toughness, generally speaking, than say, air cooling. The combination of vacuum melting plus liquid quenching has been determined as consistently providing superior results as opposed to air melting or air cooling. It will be noted that excellent impact resistance and brittle-to-ductile transition temperatures (B.D.T.T.) are obtained particularly with alloys containing above 2 percent nickel, e.g., 4 to 6 percent nickel. (The brittle-to-ductile transition temperature (B.D.T.T.) is the temperature at which the steel speci men exhibited a minimum Charpy-V notch impact resistance of 15 foot--pounds). For a high chromium ferritic stainless steel, particularly one of exceptional corrosion resistant characteristics, to absorb an impact energy of over 100 foot-pounds or more at room tempe rature and to afford a transition temperature of minus 100F. or lower, is quite unusual.

TABLE II Corrosion Behavior. Composition 10%, Fe C1 Crevice Wt. Loss.

Alloy Cr Ni Mo Al Ti Other Corrosion mgs.

F 20 2 8 na na 0.15 Si, 0.25 Mn Very Heavy 418 G 24 2 6 n.a. n.a. 0.15 Si-Mn Very heavy 8.320 15 20 2 8 0.5 0.5 0.15 Si, 0.25 Mn None 0.1 16 24 2 6 0.5 0.5 0.15 Si-Mn None 1.9 17 20 4 8 0.25 n.a. None 0.6 18 20 4 8 0.25 0.4 None 0.6 19 20 3 10 0.5 0.5 0.15 Si-Mn None 1.8

n.a. none added 0.05% Ca added in form of 307: Ca Si master alloy Balapcepf composition iign a nd i mpurities TABLE III C.\ .N,. Allo Cr Ni Mo Al Ti Others ft.-lbs. B.D T.T

No. RT. Fv

H 20 0 6 nu. 0.5 0.5 Si 9 +200 20 20 2 8 0.5 0.5 0 15 Si-Mn 24 21 20 4 8 0.5 0.5 0.15 Si-Mn 92 20 22 20 6 8 0.5 0.5 0.15 Si-Mn 144 l30 23 20 8 ii 0.5 0.5 0.15 Si Mn 124 21S 24 24 S 4 n.2i. 0.5 0 25 Si t00 25 24 6 4 0.15 0.5 151.5 1RU 26 26 5 4 n.a|. 0.5 0.25 Si l40 n.n not added In Table IV representative data concerning tensile properties of steels within the invention are given. Yield strengths well above 100,000 psi are obtainable, particularly with steels containing more than 2 percent nickel, upon aging over a temperature range of above 800 to 1,000F., e.g., 900F., for a period of about 1 to 4 hours. The data is in respect of alloys in both the annealed (2,000 or 2,lF.) and aged conditions (4 u rs at 900 F.).

sistent with good commercial steel making practice. It is, at best, most difficult to avoid the presence of carbon and to do so could add to the cost. Carbon can remove undesirable oxygen but is not essential. It is advantageous that it be kept to low levels, e.g., below 0.02 to 0.04 percent. It can be tolerated in higher amounts up to 0.1 percent but above about 0.06 or 0.07 percent, it should be fixed. It is preferred that the carbon be fixed above about -olpeiss t- ,Nit as n b2 BEES).

TABLE IV Composition Annealed Aged Alloy Cr Ni Mo Al Ti Y.S. U.T.S. El. Y.S U.T.S. EL,

% 72 '72 71 ksi ksi 7c ksi ksi 7r Each alloy deoxidized with Cu-Si nncl Si-Mn. Balance of composition iron and impurities A further attribute of the instant steels is that they possess a satisfactory fatigue life. Since it is not uncommon in assessing fatigue behavior to use a maximum fibre stress of at least equal to one-half of the tensile strength to ascertain whether a steel will perform adequately for a period of at least one hundred million cycles in duration, a smooth bar specimen of an alloy nominally containing 20 percent chromium, 6 percent nickel, 6 percent molybdenum, 0.5 percent silicon, 0.5 percent titanium, 0.15 percent calcium silicon, balance iron and impurities, was subjected to a maximum fibre stress of 58,000 psi. (The ultimate tensile strength upon annealing at 2,000F. was 105,000 psi.) Using an R. R. Moore Rotating Beam Machine it was found that the specimen exhibited a life of over 1 14,000,000 cycles at which point the tested was discontinued.

Alloys within the invention are contemplated for general use in marine environments including seawater and sea atmospheres and in connection with such operations as off-shore drilling, desalination, undersea mining, etc. More specifically, they are considered useful for pumps and parts thereof (including vanes and impellers), propellers, pipe, valves, fasteners, tubing in general including tubing for both heat exchangers and desalination equipment, tube sheets, water boxes, seawater evaporators, shafting, sheathing, marine hardware, e.g., chocks, cleats, pulleys, oil well equipment, etc., components for paper pulp equipment and chemical plant equipment for the handling of oxidizing acids and salts thereof, containers for pressure vessels for the storage and transportation of various corrosive chemicals, etc. The steels can be produced in conventional mill forms including sheet, bar, plate, rod, etc., and also as castings.

As will be understood by those skilled in the art, the term balance or balance essentially when used in referring to the iron content does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the alloy. Elements such as sulfur, hydrogen and oxygen should be maintained at low levels conent in usual amounts. Other constituents can be present as supplementary elements such as up to 4 percent tungsten and up to 10 percent cobalt. Tantalum is often found associated with columbium; however, it is heavy and costly as well as non-essential and should be held to not above 0.2 percent.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. In this regard, zirconium can be used to replace titanium, vanadium or columbium in whole or in part but it is more beneficial for various reasons to use one or more of the latter. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A stainless steel having a substantially ferritic microstructure in the ferritic condition and possessing excellent resistance to the corrosive effects of such environments as stagnant seawater in combination with good notch toughness and tensile strength, said steel consisting essentially of from 16 to 30 percent chromium, from about 3 up to about 11 percent molybdenum, silicon present up to 1.5 percent, with the proviso that the CMS indicator represented by the sum of the chromium plus three times the amount of molybde num plus three times the percentage of silicon is not less than 38 percent, nor above about 48 percent where good workability is desired, from 2 to about 8 percent nickel, up to less than 2 percent titanium, up to about 2 percent vanadium and up to about 2 percent columbium, with at least one element from the group consisting of titanium, vanadium and columbium being present in a small but effective amount to enhance the corrosion resistance in respect of steels produced by air melting and in which the CMS indicator is not above 41 percent, the sum total of titanium, vanadium and columbium not exceeding about 4 percent, up to 0.07 percent carbon, up to 2 percent aluminum, up to about 2 percent manganese, up to about 10 percent cobalt, up to about 4 percent tungsten, and the balance essentially iron.

2. A steel in accordance with claim 1 in which the CMS indicator is at least about 40 percent.

3. A steel in accordance with claim 2 in which the chromium content is from 22 to 28 percent.

4. A steel in accordance with claim 2 in which metal from the group consisting of titanium, vanadium and columbium is present in an amount of at least 0.15 percent.

5. A steel in accordance with claim 4 in which at least the element titanium is present.

6. A steel in accordance with claim 2 containing 18 to 22 percent chromium, about 5 to l0 percent molybdenum, up to about 1.5 percent silicon, a nickel content of about 25 to 6 percent, at least 0.2 percent of at least one metal from the group consisting of titanium, vanadium and columbium when the steels are produced by air melting, up to 0.5 percent aluminum, carbon present up to 0.06 percent and the balance essentially iron.

7. A steel in accordance with claim 6 in which the CMS indicator is at least 41 percent.

8. A steel in accordance with claim 2 containing from 22 to 28 percent chromium, about 3.5 to 8 percent molybdenum, up to about 1.5 percent silicon, a nickel content of about 2.5 to 6 percent, at least 0.2 percent of metal from the group consisting of titanium, vanadium and columbium when the steels are produced by air melting, up to 0.5 percent aluminum, carbon present up to 0.06 percent and the balance essentially iron.

9.A steel in accordance with claim 8 in which the CMS indicator is at least 41 percent.

10. A steel in accordance with claim 1, which has been subjected to a final annealing treatment over a temperature range of about 1,900F. to 2,200F.

H. A stainless steel containing from 16 to 30 percent chromium, from about 0.5 up to about 1 1 percent molybdenum, up to 3 percent silicon, with the provisos that the CMS indicator represented by the sum of the chromium plus three times the amount of molybdenum plus three times the percentage of silicon is not less than 34 percent, nor above about 48 percent where good workability is desired, from 2 to about 8 percent nickel. up to less than 2 percent titanium, up to about 2 percent vanadium and up to about 2 percent colum bium, with at least one element from the group consisting of titanium, vanadium and columbium being present in a small but effective amount to enhance the corrosion resistance in respect of steels produced by air melting and in which the CMS indicator is not above 38 percent, the sum total of titanium, vanadium and columbium not exceeding about 4 percent, carbon up to 0.1, up to 2 percent aluminum, up to about 2 percent manganese. up to about 10 percent cobalt, up to about 4 percent tungsten, and the balance essentially iron.

12. A steel in accordance with claim 2 in which carbon above about 0.06 percent is fixed.

13. A steel in accordance with claim 12 in which carbon above about 0.02 percent is fixed. 

2. A steel in accordance with claim 1 in which the ''''CMS'''' indicator is at least about 40 percent.
 3. A steel in accordance with claim 2 in which the chromium content is from 22 to 28 percent.
 4. A steel in accordance with claim 2 in which metal from the group consisting of titanium, vanadium and columbium is present in an amount of at least 0.15 percent.
 5. A steel in accordance with claim 4 in which at least the element titanium is present.
 6. A steel in accordance with claim 2 containing 18 to 22 percent chromium, about 5 to 10 percent molybdenum, up to about 1.5 percent silicon, a nickel content of about 2.5 to 6 percent, at least 0.2 percent of at least one metal from the group consisting of titanium, vanadium and columbium when the steels are produced by aiR melting, up to 0.5 percent aluminum, carbon present up to 0.06 percent and the balance essentially iron.
 7. A steel in accordance with claim 6 in which the ''''CMS'''' indicator is at least 41 percent.
 8. A steel in accordance with claim 2 containing from 22 to 28 percent chromium, about 3.5 to 8 percent molybdenum, up to about 1.5 percent silicon, a nickel content of about 2.5 to 6 percent, at least 0.2 percent of metal from the group consisting of titanium, vanadium and columbium when the steels are produced by air melting, up to 0.5 percent aluminum, carbon present up to 0.06 percent and the balance essentially iron.
 9. A steel in accordance with claim 8 in which the ''''CMS'''' indicator is at least 41 percent.
 10. A steel in accordance with claim 1, which has been subjected to a final annealing treatment over a temperature range of about 1,900*F. to 2,200*F.
 11. A stainless steel containing from 16 to 30 percent chromium, from about 0.5 up to about 11 percent molybdenum, up to 3 percent silicon, with the provisos that the ''''CMS'''' indicator represented by the sum of the chromium plus three times the amount of molybdenum plus three times the percentage of silicon is not less than 34 percent, nor above about 48 percent where good workability is desired, from 2 to about 8 percent nickel, up to less than 2 percent titanium, up to about 2 percent vanadium and up to about 2 percent columbium, with at least one element from the group consisting of titanium, vanadium and columbium being present in a small but effective amount to enhance the corrosion resistance in respect of steels produced by air melting and in which the ''''CMS'''' indicator is not above 38 percent, the sum total of titanium, vanadium and columbium not exceeding about 4 percent, carbon up to 0.1, up to 2 percent aluminum, up to about 2 percent manganese, up to about 10 percent cobalt, up to about 4 percent tungsten, and the balance essentially iron.
 12. A steel in accordance with claim 2 in which carbon above about 0.06 percent is fixed.
 13. A steel in accordance with claim 12 in which carbon above about 0.02 percent is fixed. 